Using errno in C

Introduction

errno helps you find out what went wrong when a C function fails. It stores error codes so you can understand and fix problems.

When a function like fopen fails to open a file and you want to know why.

When a system call returns an error and you need the error reason.

When debugging your program to see what caused a failure.

When you want to show a helpful error message to the user.

When handling errors in functions that set errno on failure.

Syntax

C

#include <errno.h>
#include <stdio.h>
#include <string.h>

// errno is a global variable
// Use perror() or strerror(errno) to print error messages

errno is set by system and library calls when an error happens.

Do not change errno yourself; just read it after a failure.

Examples

This example tries to open a file that does not exist and prints the error message using strerror(errno).

C

#include <errno.h>
#include <stdio.h>
#include <string.h>

int main() {
    FILE *f = fopen("missing.txt", "r");
    if (!f) {
        printf("Error opening file: %s\n", strerror(errno));
    }
    return 0;
}

Here, perror prints a message with the error description automatically.

C

#include <errno.h>
#include <stdio.h>

int main() {
    FILE *f = fopen("missing.txt", "r");
    if (!f) {
        perror("fopen failed");
    }
    return 0;
}

Sample Program

This program tries to open a file that does not exist. It then prints the numeric error code, the error message string, and uses perror to show the error.

C

#include <errno.h>
#include <stdio.h>
#include <string.h>

int main() {
    FILE *file = fopen("nonexistent.txt", "r");
    if (!file) {
        printf("Failed to open file. errno = %d\n", errno);
        printf("Error message: %s\n", strerror(errno));
        perror("perror output");
        return 1;
    }
    fclose(file);
    return 0;
}

Important Notes

errno is only valid right after a function fails; calling other functions may change it.

Always check the function's return value before using errno.

Different systems may have different error codes, but common ones like ENOENT (file not found) are standard.

Summary

errno stores error codes set by failed functions.

Use strerror(errno) or perror() to get readable error messages.

Check function results before reading errno.

Practice

1. What is the primary purpose of errno in C programming?

easy

A. To store error codes set by system or library functions when they fail

B. To count the number of errors in a program

C. To reset the program state after an error

D. To display error messages automatically

Solution

1. Step 1: Understand what errno stores

   errno is a global variable that stores error codes set by system or library functions when they fail.

2. Step 2: Differentiate from other options

   It does not count errors, reset program state, or display messages automatically.

3. Final Answer:

   To store error codes set by system or library functions when they fail -> Option A

4. Quick Check:

   errno stores error codes = C [OK]

Hint: Remember: errno holds error codes after failure [OK]

Common Mistakes:

• Thinking errno counts errors
• Assuming errno resets program state
• Believing errno prints messages automatically

2. Which of the following is the correct way to include errno in a C program?

easy

A. #include

B. #include

C. #include

D. #include

Solution

1. Step 1: Identify the standard header for errno

   The standard header file to use errno is <errno.h>.

2. Step 2: Verify other options

   <error.h>, <err.h>, and <errors.h> are not standard headers for errno.

3. Final Answer:

   #include <errno.h> -> Option B

4. Quick Check:

   errno header = <errno.h> [OK]

Hint: Use #include to access errno [OK]

Common Mistakes:

• Using wrong header files
• Forgetting to include errno.h
• Confusing with similar-sounding headers

3. What will the following C code print if the file nonexistent.txt does not exist?

#include <stdio.h>
#include <errno.h>
#include <string.h>

int main() {
    FILE *f = fopen("nonexistent.txt", "r");
    if (!f) {
        printf("Error: %s\n", strerror(errno));
    }
    return 0;
}

medium

A. Error: No such file or directory

B. Error: Permission denied

C. Error: File exists

D. Error: Unknown error

Solution

1. Step 1: Understand fopen failure and errno

   When fopen fails to open a non-existing file for reading, it sets errno to indicate the error, usually ENOENT meaning "No such file or directory".

2. Step 2: Use strerror to get error message

   strerror(errno) converts the error code to a readable string, which will be "No such file or directory".

3. Final Answer:

   Error: No such file or directory -> Option A

4. Quick Check:

   fopen missing file error = "No such file or directory" [OK]

Hint: fopen missing file sets errno to ENOENT message [OK]

Common Mistakes:

• Assuming errno is zero without checking fopen result
• Confusing error messages for different errors
• Not including string.h for strerror

4. Identify the error in this code snippet that uses errno:

#include <stdio.h>
#include <errno.h>
#include <string.h>

int main() {
    errno = 0;
    FILE *f = fopen("file.txt", "r");
    if (f == NULL) {
        perror("File open error");
    }
    if (errno != 0) {
        printf("Error code: %d\n", errno);
    }
    return 0;
}

medium

A. Using perror instead of strerror is incorrect

B. Setting errno to 0 before fopen is unnecessary

C. Checking errno after fopen without checking fopen result first

D. Not closing the file pointer after fopen

Solution

1. Step 1: Analyze errno usage after fopen

   errno should only be checked if the function indicates failure. Here, errno is checked regardless of fopen result, which can be misleading.

2. Step 2: Confirm correct error checking pattern

   The correct pattern is to check if fopen returned NULL before reading errno. This code checks errno even if fopen succeeded.

3. Final Answer:

   Checking errno after fopen without checking fopen result first -> Option C

4. Quick Check:

   Check function result before errno [OK]

Hint: Always check function result before reading errno [OK]

Common Mistakes:

• Reading errno without function failure
• Confusing perror and strerror usage
• Ignoring file close after fopen

5. You want to open a file for reading and print a custom error message if it fails. Which code snippet correctly uses errno and strerror to do this?

A) FILE *f = fopen("data.txt", "r");
   if (!f) {
       printf("Error: %s\n", strerror(errno));
   }

B) errno = 0;
   FILE *f = fopen("data.txt", "r");
   if (f == NULL) {
       printf("Error: %s\n", strerror(errno));
   }

C) FILE *f = fopen("data.txt", "r");
   if (f == NULL) {
       errno = 0;
       printf("Error: %s\n", strerror(errno));
   }

D) FILE *f = fopen("data.txt", "r");
   if (f == NULL) {
       perror("Error opening file");
   }

hard

A. Option A

B. Option B

C. Option C

D. Option D

Solution

1. Step 1: Check correct error handling with errno

   Option D uses perror, which is the standard and simplest way to print a custom error message along with the error string corresponding to errno. It automatically handles errno correctly.

2. Step 2: Identify issues in other options

   Option D does not reset errno before fopen, which is less safe. Option D resets errno before fopen and checks failure, but it's generally unnecessary to reset errno before calls that set it on failure. Option D resets errno after failure, clearing the error code before printing, which is incorrect.

3. Final Answer:

   Option D -> Option D

4. Quick Check:

   Use perror for simple and correct error messages [OK]

Hint: Use perror for automatic error message printing [OK]

Common Mistakes:

• Resetting errno unnecessarily
• Resetting errno after failure
• Using strerror without valid errno